1. Field of the Invention
The present invention relates generally to satellite communications. More particularly, the present invention relates to a method and system of satellites which are capable of providing continuous unbroken links between satellites disposed at different altitudes, such as a satellite in Medium Earth Orbit and a satellite in Geosynchronous orbit.
2. Description of the Prior Art
Communication and data satellite relays are used to facilitate the transfer of information between two locations. The locations could be satellites or ground stations. These "users" may need to use a relay method due to any of the following reasons: (1) the earth is between the user and a location with which the user wishes to communicate; (2) the user cannot point in the direction of its intended location, but is able to point to a relay; (3) the user does not have the power or equipment to communicate directly with the desired remote location. The principal factors of consideration for satellite relays are the following: (1) cost; (2) effectiveness (such as, data rate, coverage, number of users, and so forth); (3) complexity; (4) the burden on the user in terms of the amount of power and specialized equipment which is required by the user to communicate with the satellite relay; (5) transition from old relay systems to a new one; (6) and flexibility.
Geostationary satellites may be used as relay stations since the geostationary satellites would be stationary over the earth and thus the ground stations could use a fixed antenna. Their advantage is that ground users see a relatively low change in the line-of-sight (LOS) from the users to the geostationary satellite relay. Also, geostationary satellites are more difficult for an adversary to destroy due to the greater energy required of the intercept vehicle and the long warning time.
However, geostationary satellites are not without disadvantages in a communication relay application. They require large boosters to launch to the high geostationary altitude of 22,300 miles (35,900 km) above the equator; and due to the high altitude, they require large antennas to transmit and receive radio frequency (RF) signals. Likewise, users are required to carry larger antennas to transmit and receive signals with a satellite at the geostationary altitude.
Other disadvantages with geostationary satellite relays are that the geostationary orbit is relatively crowded as it extends around the equator and at least 3 geostationary satellites would be required to cover most ground stations. Also, spares are expensive, and an inclined-orbit would be needed to provide service to the ground high latitudes. The use of a geosynchronous satellite with its inclined orbit would virtually eliminate the stationary, fixed user antenna advantage and would require more satellites to provide good coverage to all latitudes.
In general, geostationary satellites with any type of satellite link suffer these disadvantages. In the field of the present invention, a satellite link entails an unidirectional or bidirectional transmission of information between a satellite and another entity via radio frequency, laser, or other suitable signal conveying techniques. The other entity may be another satellite, a ground station, or some other entity capable of transmitting or receiving signals through space, such as an airplane or space station.
Satellite relays at lower altitudes could also be considered, such as Low Earth Orbit (LEO), Medium Earth Orbit (MEO), or Highly Elliptical Orbit (HEO). However, lower altitude satellite relays require many more satellites to cover the earth to provide continuous service. When more than one satellite relay is used in coordination to cover the earth, the multiple satellite relays are collectively termed a "constellation".
Lower altitude satellite relays are also more vulnerable to attack and have relatively high angular LOS rates and more frequent "make/Lreak" requirements. HEO satellites operate most of the time near a geosynchronous altitude, with many of the same advantages and disadvantages of geosynchronous satellites. HEO links must be made and broken, although less frequently than at LEO or MEO altitudes. Lastly, to link with a ground station, the lower altitude satellite relays usually need to establish communication links with other satellite relays in their constellation. This cross-linking among other satellite relays in the constellation results in a complex series of routing and making and breaking of communication links.
These systems route the signal through the use of cross-links, which are intelligent switches and routers on board each satellite, among many satellites. This "intelligent switch/router" in space is a major drawback since these satellite systems have to point at other satellites and have the complexity of determining who should be the recipient satellite of the message. The situation becomes even more complex if the selected recipient satellite is overloaded with other data transmissions. Such a situation forces the sending satellite to reroute the message to another satellite.
Therefore, it is an objective of the present invention to provide a satellite relay system that is less complex, more flexible, more robust to potential problem conditions, and lowers the user burden to perform the relay function.
It is another objective of the present invention to provide a satellite relay system in which cross links are not interrupted, and therefore avoids the need for a first satellite to break its communication link with a second satellite and to establish a new link with a third satellite as the second satellite moves behind the earth relative to the first satellite.
Another objective is to provide uniterrupted cross links that are always above the horizon, i.e., the line of sight or its extension never intersects the earth.
It is a further objective of the present invention to provide a satellite relay system that consumes less power by requiring smaller satellites to act as satellite relays of RF or laser signals.